1. Field of the Invention
This invention relates to a set of color developers for use in multi-color electrophotography, particularly suitable for use in digital multi-color electrophotography, and a multi-color electrophotographic image formation method using the same, capable of producing high quality glossy images.
2. Discussion of Background
Although various electrophotographic image formation methods are disclosed, for example, in U.S. Pat. No. 227,691, and Japanese Patent Publications 42-23910 and 43-24748, the most common method is such that a latent electrostatic image is first formed on the surface of a photoconductive support by using a photoconductive material and then developed with a developer (toner) to obtain a visible image, or a toner image is transferred to a sheet of paper or the like and then fixed thereon by application of heat or pressure, or by the use of a vaporized solvent to obtain visible images.
A multi-color electrophotographic image formation method of an analogue type is disclosed, for example, in U.S. Pat. No. 2,962,374. In this method, the colors of an original image are separated and converted into electric signals corresponding to at least three colors of yellow, magenta and cyan, and the surface of a photoconductor is electrically charged in accordance with each of the color signals to form a latent electrostatic image corresponding to each of the separated colors, and each of the latent images is then successively developed with a corresponding color developer of yellow, magenta or cyan to obtain multi-color images.
In addition, various proposals are now being made on a multi-color electrophotographic image formation method of a digital type. In this method, an original image is subjected to color separation to obtain color signals, which are then subjected to an A/D conversion to obtain digital color signals. By conducting a predetermined computation, recording signals are obtained from the digital color signals, and latent electrostatic images are formed on the surface of a photoconductor in accordance with the digital signals. The latent images are then developed with color developers to obtain visible multi-color images.
As described above, in the digital multi-color electrophotographic image formation method, the optical information is subjected to computation, so that the masking treatment and the UCR (undercolor removal) treatment can be carried out, which cannot be successfully carried out in the conventional multi-color electrophotographic image formation method of an analogue type.
In the case where the UCR treatment is conducted, a latent image corresponding to black color can be developed with a black developer to obtain a black image, instead of successively overlapping yellow, magenta and cyan toner images. A gray balance of the obtained images can thus be improved; in other words, excellent half-tone images are obtainable. In addition, the thickness of the toner layer of the black area obtained by using a black developer is thinner than that of the black area obtained by super-imposing yellow, magenta and cyan images. The digital method is therefore economically advantageous, and can provide images without the problem of the curling of a transfer sheet.
A black developer and color developers of yellow, magenta and cyan are required to have high chargeability, low environmental dependency and high fixing ability. It is also required that they be not deteriorated even when they are repeatedly used for a long period of time. In addition to the above, the color developers are required to have good characteristics in terms of the spectral reflectance, high transparency, and capabilities of exhibiting bright mixed-color and to producing images having a proper degree of glossiness.
In order to fulfill the above requirements, a resin having relatively low softening point and high transparency is used as a binder resin in the color developers for use in the conventional multi-color electrophotography of an analogue type.
In the case where such a binder resin is incorporated into a black developer for use in the digital multi-color electrophotography in which the UCR treatment is conducted, the viscoelasticity of the black toner is almost equal to that of each of the color developers of yellow, magenta and cyan.
It is well known that the glossiness of the obtained images varies depending upon the viscoelasticity of the developer and the thickness of a toner layer.
When multi-color images are formed by the digital method using the black and color developers of magenta, yellow and cyan, the toner layer of a black area is thinner than that of a secondary color (red, green, blue) area. As a result, the glossiness of the black area is lower than that of the other areas. Uneven glossiness brings about low image quality. In particular, the glossiness of a black area has psychological importance. Therefore, it is necessary to enhance the glossiness of the black images to at least as high as that of the images in other colors.